This invention relates in general to renewable electrochemical energy storage by redox flow battery systems and more in particular to vanadium redox secondary batteries.
Electrochemical systems because of their theoretically high efficiency have long been looked at as ideal energy conversion systems. In particular secondary batteries are by definition extremely interesting candidates for energy storage systems. Load levelling and peak-shaving in electric power generation, distribution and use are all areas where secondary batteries may offers very efficient solutions.
Among secondary batteries, the so-called redox flow battery or more briefly redox (cells) batteries employ solutions for storing the energy; the cell hardware simply providing an appropriate support for the parallel reduction and oxidation (redox) half-cell reactions, during both modes of operation, that is during the charging and the discharging processes.
The use of redox couples of the same (multivalent) element, that is for the a negative electrode redox couple as well as for the positive electrode redox couple, offers a great simplification in the handling and storage of the dissolved species.
The vanadium redox flow battery also referred to as the all-vanadium redox cell or simply the vanadium redox cell or battery, employs V(II)/(III) and V(IV)/(V) as the two redox couples, in the negative (sometime referred to as the anolyte) and positive (sometime referred to as the catholyte) half-cell electrolyte solutions, respectively.
Numerous publications on the all-vanadium redox cell have recently been published. Among these, the following provide an update overview of the secondary battery field, also including comparative cost analysis with alternative renewable energy storage systems, as well as among the most promising redox flow batteries that are being developed.
GB-A-2,030,349-A discloses a process and an accumulator for storing and releasing electrical energy based on a solid polymer electrolyte flow redox battery; Chromium-chromium redox couples and Vanadium-vanadium redox couples being indicated as viable choices.
U.S. Pat. No. 4,786,567, EP-A-0,517,217-A1, U.S. Pat. Nos. 5,250,158, 5,318,865, as well as the following articles:
xe2x80x9cImproved PV System Performance Using Vanadium Batteriesxe2x80x9d by Robert L. Largent, Maria Skyllas-Kazacos and John Chieng, Proceedings IEEE, 23rd Photovoltaic Specialists Conference, Louisville, Ky., May 1993;
xe2x80x9cElectrochemical Energy Storage and Vanadium Redox Batteryxe2x80x9d by Maria Skyllas-Kazacos, unpublished article freely distributed for general information purposes;
xe2x80x9cThe Vanadium Redox Battery for Efficient Energy Storagexe2x80x9d by Maria Skyllas-Kazacos, unpublished article freely distributed for general information purposes; and
xe2x80x9cStatus of the Vanadium Redox Battery Development Programxe2x80x9d by C. Menictas, D. R. Hong, Z. H. Yan, J. Wilson, M. Kazacos and M. Skyllas-Kazacos, Proceedings Electrical Engineering Congress, Sydney, November 1994;
are all pertinent to the so called xe2x80x9cVanadium Redox Systemxe2x80x9d.
The publication WO 95/12219 describes methods for preparing stabilized solutions of vanadium and related redox systems.
EP-A-0,566,019-A1 describes a method for producing vanadium electrolytic solutions.
WO 95/17773 describes a combined system for producing electric energy in a biofuel cell, based on a vanadium redox flow system.
Typically and in general a redox flow battery systems includes two separate tanks, namely a catholyte tank and an anolyte tank and a plurality of cell stacks or batteries.
The capacity of the two tanks must be sufficient to provide for the required renewable energy storage capacity.
The overall cell area and the number of cells must be such as to satisfy the peak current and the xe2x80x9cnominalxe2x80x9d DC voltage requisites, respectively, thus dictating the electrical configuration (series and/or parallel) of the plurality of stacks or batteries.
The two hydraulic circuits of the catholyte and of the anolyte, respectively, is must be substantially separated from one another, each having its own circulation pump or pumps.
In a system employing single catholyte and anolyte tanks, that is functioning in a recirculation mode, the catholyte and the anolyte flow through the respective compartments of the unit cells of each stack or battery. Depending on whether the secondary battery is being discharged by flowing a current in an external electrical circuit that includes an electrical load, or being charged by forcing a current through the battery, both the catholyte and the anolyte are respectively discharged or charged.
Conventionally, a positive half-cell electrolyte solution (catholyte) is said to be charging when the redox couple therein is being oxidized more and more to the higher of the two valence states and to be discharging when the redox couple therein is being reduced more and more to the lower of the two valence states. Conversely, a negative half-cell electrolyte solution (anolyte) is said to be charging when the redox couple therein is being reduced more and more to the lower of the two valence states and to be discharging when its redox couple is being oxidized more and more to the higher of the two valence states.
As an alternative, instead of been operated in a recirculation mode, a redox flow system may be operated in a xe2x80x9cbatch modexe2x80x9d.
According to this alternative mode of operation, both the negative half-cell electrolyte circuit and the positive half-cell electrolyte circuit include two tanks, respectively for the relatively spent or discharged solution and for the relatively charged solution. Pumps will be commanded to pump the positive half-cell electrolyte and the negative half-cell electrolyte from their respective spent electrolyte tanks to their respective charged electrolyte tanks during a charging phase of the battery and viceversa, when the battery is operated as an electrical energy source, to invert the direction of flow of the negative half-cell electrolyte and of the positive half-cell electrolyte streams so that the solutions be flown from the respective charged solution tanks to the respective spent solution tanks.
The batch mode of operation provide for a xe2x80x9cvolumetricxe2x80x9d indication of the state of charge or of discharge of the system.
The stacks or batteries of individual cells comprise a plurality of cells in electrical series defined by a stacked repetitive arrangement of a conductive intercell separator having a generally bipolar function, a positive electrode, an ion exchange membrane, a negative electrode and another conductive intercell separator.
Each electrode is confined in a flow compartment, usually having an inlet manifolding space and an outlet manifolding space.
The actual voltage of each unitary redox flow cell during discharge when an electrical load is connected as well as the voltage that is needed to force a current through the cell during a charging phase, depends on the specific half-cell reactions (basically on the redox couple been used), however such a standard cell potential will be diminished during discharge and increased during charge by the energy losses associated with the internal resistance (R) of the cell, the overvoltage losses due to the finite kinetic of the half-cell reactions (activation overvoltage: xcex7a)) and the mass transport limitations (concentration overvoltage: xcex7c).
In practice, the actual voltage needed to charge the battery and the voltage delivered by the battery during discharge (charge), will be given in first approximation by the following equations:
Eocell=Eocathodexe2x88x92Eoanodexe2x88x92iRxe2x88x92naxe2x88x92nc 
Eocell=Eocathodexe2x88x92Eoanode+iR+na+nc 
While the terms Eocathode and Eoanode representing the standard half-cell potentials will depend on the state of charge of the positive half-cell electrolyte and of the negative half-cell electrolyte besides temperature, the other terms reflect the kinetic limitations of the electrochemical reactions and the ohmic losses through the cell.
Redox flow batteries are customarily realized in the form of xe2x80x9cbipolarxe2x80x9d stacks that may include up to several hundred unit cells in electrical series. However, the largest is the number of unit cells that are stacked together the more critical becomes dimensional and planarity tolerances of construction and hydraulic sealing of such a large number of bipolar elements assembled together in a xe2x80x9cfilter-pressxe2x80x9d arrangement may become problematic.
Moreover, considering that the negative half-cell electrolyte and the positive half-cell electrolyte are circulated in parallel through all the respective flow compartments of the stack by conventionally constituting inlet and outlet manifolds by assembling together cell frames, electrodes, membranes and gaskets all provided with aligned holes, electric current by-pass along the body of electrolytes contained in these manifolds that extends along the entire length of the stack, become extremely critical in view of the large voltages involved.
By-pass current in the stack""s manifolds may cause severe pitting corrosion phenomena on (half-cell) discharging surfaces and even where corrosion is not induced, they contribute to lower the overall faradic efficiency of the redox system.
Another typical behavior of redox flow battery systems, irrespectively of whether they are operated in a recirculation mode or in a batch mode, is represented by the fact that the standard cell potential is not relatively constant but varies significantly depending on the state of charge of both the negative half-cell electrolyte and the positive half-cell electrolyte. This standard cell potential variation during a peak-shaving or load-levelling application of the redox system creates nonnegligible problems of optimization of the electrical hardware of the renewable energy storage system. These problems normally require implementation of a microprocessor-based control and a remarkable complication of the inverters circuitry in order to compensate for the declining battery voltage during a discharge phase and for a cell voltage increase during a charge phase.
These problems are particularly relevant in all-vanadium redox batteries because of the relatively large variations of the standard half-cell potentials that are observed.
It has now been found and represents the object of the present invention, an improved method of operating a redox flow battery system that alleviates or completely eliminate the above-noted problems and drawbacks of the known systems.
Essentially, the method of the invention is based on flowing the negative half-cell electrolyte and the positive half-cell electrolyte through the respective compartments of a battery stack in cascade rather than in parallel as customarily implemented in prior art batteries.
It has been found that by circulating the negative half-cell electrolyte and the positive half-cell electrolyte solution in cascade or in sequence from the respective compartment of a first cell to the respective compartment of the next cell of the stack and so forth to the compartment of the last cell of the stack, by-pass currents in the stack may be almost completely eliminated. In practice only a negligible residual cell-to-cell by-pass path remains on which will insist the voltage of a single cell, irrespectively of the number of cells of the battery. Such a relatively small in consideration of the electrical resistance of the liquid body present in the hydraulically connecting conduit will produce a negligible residual level of by-pass current and will not cause any appreciable corrosion.
Furthermore, electric current path interruptions may be easily implemented outside the stack, most preferably at the respective tank inlet or even along the hydraulic circuit, between stacks. Electric path interruptions in the liquid xe2x80x9cveinxe2x80x9d constituted by the ducted stream of electrolyte may be implemented by employing a single or multilevel drip column. The system of the present invention permits to install such a current interruption device at the inlet of a storage tank and conveniently even inside the tank itself, in a top (non flooding) vent-portion thereof.
It has been found that any increased pumping requirement is more than compensated by the improved faradic efficiency of the electrochemical processes during charging and discharging phases.
Moreover, an appropriate design of the flow compartments of the cells can dramatically reduce the pumping requirements, that is the pressure drop along the cascade of compartments of a stack or of a plurality of stacks hydraulically fed in cascade, as will be illustrated later in this description.
The method of the invention is applicable irrespectively of the fact that the redox flow battery system be operated in a recirculation mode, employing only two distinct tanks, one for the negative half-cell electrolyte solution and the other for the positive half-cell electrolyte solution, or in a batch mode by employing two pairs of tanks, one pair for the positive half-cell electrolyte solution and the other pair for the negative half-cell electrolyte solution.
The two streams of negative half-cell electrolyte and positive half-cell electrolyte may be fed parallel into the respective flow compartments of a first cell of the stack (or of a first stack of a plurality of stacks in cascade) and flown in cascade up to the respective compartments of the last cell of the stack (or of the last of the stacks) to be eventually recycled to the respective tanks.
This mode will reproduce substantially the same half-cell conditions that are normally present in conventionally operated flow redox battery, whereby the voltage contribution of each cell of the stack (or of the plurality of stacks) electrically connected in series, will be determined, nominally, from the actual state of charge of the positive half-cell electrolyte and of the negative half-cell electrolyte solution present in the cell.
According to a preferred alternative embodiment of the method of operation of the invention, the negative half-cell electrolyte and positive half cell electrolyte streams are respectively fed into the respective compartment of a first cell, at one end and at the opposite end of the stack (or of the plurality of stacks connected in electrical series) of the cells in electrical series and therefore passed along the plurality of individual cells in electrical series in a xe2x80x9ccounter-currentxe2x80x9d mode.
In this way, conditions are established whereby the first cell at one end of the electrical series will function with a relatively charged negative half-cell io electrolyte or positive half-cell electrolyte and with a relatively discharged positive half-cell electrolyte or negative half-cell electrolyte and the last cell at the other end of the electrical series will be functioning with a reversed relative charge condition of the two electrolytes.
According to such an alternative embodiment, the method of the invention offers important and unsuspectable advantages.
A first advantage is represented by the fact that the method of circulation of the invention may be exploited to implement a self-averaging mechanism on a time-base (that is during the time taken by a given volume of negative half-cell electrolyte and of positive half-cell electrolyte to pass through the battery) of the nominal voltage produced (in a discharge phase) at the end terminals of a stack (or of a plurality of stacks connected in electrical series).
It has been found that by so counter balancing the relative state of charge of the positive half-cell electrolyte and or the negative half-cell electrolyte through the plurality of cells of a single stack or of the plurality of stacks connected in electrical series, the magnitude of variation of the nominal cell voltage that is mainly imputable to the progressive discharging or charging of the negative half-cell electrolyte and of the positive half-cell electrolyte solution may be substantially reduced, thus alleviating the problems of compensating for such a marked decline or rise of the cell voltage respectively during a discharge phase and during a charge phase.
In load-levelling and peak-shaving applications this time-base averaging mechanism of the battery voltage may be decisive in greatly simplifying the electrical circuitry design and management by simply reducing cell voltage excursions.
An additional important advantage of the circulation method of the invention, when implemented in a xe2x80x9ccounter currentxe2x80x9d mode, is a significant reduction of the phenomenon of water transfer unbalance through the ion exchange membranes that separate the respective negative half-cell electrolyte and positive half-cell electrolyte compartments of each individual cell.
As it is well known, redox flow battery systems are somewhat plagued by such a phenomenon that produces an increase of the volume of either the positive half-cell electrolyte or the negative half-cell electrolyte while proportionally decreasing the volume of the other. This phenomenon requires periodic re-equalization of the volumes of the negative half-cell electrolyte and of the positive half-cell electrolyte in their respective circuits.
In an all-vanadium redox flow battery system, a net water transfer from the positive half-cell electrolyte compartment to the negative half-cell electrolyte compartment is observed when the ion exchange separator is an anionic membrane while when a cationic membrane is used a reversed net water transfer from the negative half-cell electrolyte to the positive half-cell electrolyte is observed.
It is also accepted that the water transfer through the ion exchange membrane in the form of the hydration shells of the migrating ionic species is little significant as compared with the amount of water been transferred by osmosis.
The method of operation of the invention reduces the net water transfer through the membrane by reducing the concentration gradient across the membrane, during discharge and change phases.
According to a further aspect of this invention, the phenomenon of unbalanced water transfer may be practically eliminated by alternately installing a cation exchange (cationic) membrane and an anion exchange (anionic) membrane for separating the flow compartments of the single cells in every stack or battery or installing all cationic membranes in one stack and all anionic membranes in a second stack, and so forth. The opposite xe2x80x9cdirectionxe2x80x9d of the net unbalancing water transfer during the is cycling of the battery or batteries, as determined by the different kind of ion-selective cell separator installed, will decisively help to curb this undesired phenomenon to be practically negligible.
Moreover, the peculiar cascade circulation of the electrolytes, according to this invention, makes possible another yet utterly resolutive technique for completely overcoming the problem of unbalanced water transfer that otherwise would be impracticable in the operation modes of the prior art because of an unbearable accompanying loss of efficiency.
Under particular but recurrent conditions of operation, and precisely in systems operated in a batch mode and designed for a cycling of the batteries that includes a phase of substantially complete discharging of the negative half-cell electrolyte and positive half-cell electrolyte solutions after a similarly protracted phase of charging, as for example in a day-time exploitation of recoverable energy stored during the night, in a battery installation operated according to a cascade and counter current mode of circulation of this invention, the xe2x80x9cspentxe2x80x9d solution tanks for the negative half-cell electrolyte and the positive half-cell electrolyte may be unified in a single tank.
In practice, upon termination of any full discharge phase of operation, a volumetric equalization is practically implemented. During the charging process, the electrolyte recovered in the single tank is pumped into separate streams of negative half-cell electrolyte and positive half-cell electrolyte through the batteries to the respective tanks where the charged negative half-cell electrolyte and positive half-cell electrolyte solutions may be stored separately. In a battery installation of the prior art, implementing a parallel feed of the homopolar flow compartments of a battery or even in an installation implementing a cascaded flow through the homopolar compartments but in an equicurrent mode, unification of the two electrolytes, even if done with substantially discharged electrolytes, will determine a loss of efficiency that would remain prohibitive.
This can be easily recognized by considering for example that, in the case of an all vanadium battery, a completely discharged positive half-cell electrolyte will contain ideally all the vanadium as V(IV) because all the V(V) initially present in the charged solution can be reduced to just V(IV). Similarly, a completely discharged negative half-cell electrolyte will contain ideally all the vanadium as V(III) because all the V(II) initially present in the charged solution can be oxidized to just V(III).
If the two completely discharged electrolytes were to be mixed together, a solution containing about 50% of V(III) and 50% of V(IV) would be obtained. As a consequence, during the successive charging phase, a conspicuous amount of energy would have to be spent at the beginning in order to re-oxidize the 50% content of V(III) to V(IV) in the positive half-cell electrolyte, before starting to build up exploitable charge (to V(V)), and reduce back to V(III) the 50% content of V(IV), before starting to build up exploitable charge (to V(II)). In other words, mixing together the two spent electrolytes (with the objective of re-equalizing their circulating volumes) entails a major loss of charge (efficiency).
By contrast, operating in a counter current mode, that is with substantially xe2x80x9casymmetricxe2x80x9d conditions, during a full discharge process, it is possible to xe2x80x9cover-reducexe2x80x9d the vanadium in the positive half-cell electrolyte to become a mixture of V(IV) and V(III) and to xe2x80x9cover-oxidizexe2x80x9d the vanadium in the negative half-cell electrolyte to become a mixture of V(III) and V(IV). This is made possible because toward one end of the stack, the xe2x80x9cover-reducingxe2x80x9d solution of V(IV) and V(III) in a positive half-cell compartment of a cell will confront itself with a relatively charged solution still containing a large proportion of V(II) as compared with the content of V(III) in the negative half-cell compartment of the cell and similarly, toward the opposite end of the stack, the xe2x80x9cover-oxidizingxe2x80x9d solution of V(III) and V(IV) in a negative half-cell compartment will confront itself with a solution still containing a large proportion of V(V).
Therefore, when the two streams are unified in a single spent electrolyte tank, only a residual difference will exist between the two incoming streams and their mixing together will entail only a residually small loss of (charge) efficiency. Such a residually small loss of efficiency will be more than compensated by the automatic requalization of the two circulating volumes of electrolytes. In any case re-equalization of the unbalanced volumes, even if done periodically as in the known systems, inevitably causes a loss of charge much larger than in a system operated according to the above embodiment of the method of this invention.
Moreover, the above method has the attendant advantage of practically allowing for an energy storing capacity that can be as large as 50% in excess than that possible according to the prior art for the same amount of vanadium employed. Altogether, the investment per unit of energy of storage capacity will be substantially decreased.